Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss	$ (49,876)	$ (37,565)	$ (31,218)	$ (30,229)	$ (87,441)	$ (61,447)
Denominator:
Weighted average shares of common stock outstanding (in shares)	41,274,808		30,944,899		41,261,808	30,915,577
Net loss per share, basic and diluted	$ (1.21)		$ (1.01)		$ (2.12)	$ (1.99)